Automatic frequency control



June 20, 1,939. N. P. CASE AUTOMATIC FREQUENCY CONTROLv Filedpril '3, 193s 2 sheets-'sheet 1 ATTORNEY.

June 20, 1939. N. P. CASE 2,163,234

AUTOMATIC 'FREQUENCY CQNTROL n Filed April/gg 1936 2 Sheets-Sheet 2 Reodm .l 2 Mlstuning QDizgl Change *5 htlOVrON- (an) sagouanbalj seguo() awgpauuaw 4o suoungAaq FIG. 2.

1N VENTOR.

ATTORNEY.

Patented .im zo, `1939 `v f UNITED STATES PATENT OFI-'luca 2,163,234 Y AU'roMA'rTc FREQUENCY ooN'raoL Nami P. case, Great Neck, n. v .:mgnor u Hazeltine Corporation, a corporation of Delawill .Application April 3, 1936, Serial No.V 72,532

4 'nus invention relates to selective modulatedi can-ler systems of the superheterodyne A 'tion usually extending 5 or more kilocycles on quency of the signal developedby the frequencyeither side thereof. 'Ihe various broadcasting stations are allotted different carrier frequencies which, in the present practice, are uniformly spaced throughout the broadcast frequency range, the spacing of adjacent carrier frequencies usually being 10 lrilocycles.-

' In a conventional 'superheterodyne receiver, there is included a frequency changer tunable over a wide range of frequencies for deriving from any desired modulated-carrier signal within an equal related range a second modulatedcarrier signal normally having a. predetermined carrier frequency. An intermediate-frequency -chanrl is coupled to the output of the frequency-changing means, designed selectively to pass this predetermined carrier frequency and its s'idebands of modulation. For optimum selectivity and fidelity of reception the frequency of the modulation carrier developed by the frequency changer should be located substantially at the center frequency of'the intermediate-frequency channel, that is, it` should always be maintained at the said predetermined frequency. As is welt understood in the art', however, any mistuning of the frequency-changing means, due to incorrect timing adjustments by the operator, to drift in the frequency of the oscillations produced in the frequency changer, or to other causes, products deviations inthe carrier frechanging means from its normal or'predetermined frequency, that is, from the mean frequency-of the intermediate-frequency channel,

thereby impairlng the selectivity and dellty of reception to the extent of such deviations.-

Certain arrangements have heretofore been devised in attempts to control the frequency of the signal derived by the frequency-changing4 means, thereby automatically to reduce 'these deviations. Satisfactory automatic frequency control for reducing the mentioned deviations, however, A hasV vaous operating characteristics which, heretofore, have not been procured. Ammg these characteristics are the following:

The intermediate carrier frequency, when undu' control, should be heldV within relatively (ci. vzar-2o) narrow limits of deviation, for example, 11000 cycles from the desired predeterminedfrequency. 'l'he automatic-control action should be effective for mistuning of the frequency changer to an extent of the order of, but less than, the frequency separation of the signals in Vthe broadcast range, and ineffective or relaxed, or preferably reversed in its action, for greater extents of mistuning, Vin order that-the frequencychanger may thereupon immediately be adjusted to receive thev next adjacent signal in the range.

Moreover, the frequency control action should have equal sensitivity and range throughout the4 range of frequencies to which the system is tunable, and such control action should b e independent of the signalstrength of any usable received signal, as well as independent of the ordinary variations in the operating voltages utilized for the system. Various other characteristics must be achieved in order that the system shall be satisfactory from the standpoint of stability of operation and shall be commercially feasible in its construction, as will more apparent hereinafter.

The object of the present invention, therefore, is to provide an improved automatic frequency control for the frequency changer of a modulated-carrier signaling system embodying one or more of the desirable characteristics set. forth above.

In accordance with the present invention, there is provided a selective Vmodulated-carrier Sig- Vdeviationswithin narrow limits vupon mistumng of the frequency-*changing means. 'I'his tunable frequency-changing means may comprise a smgie tunable frequency changer or two or-more frequency changers coupled in cascade only the Frequency-discrimi- Yhating means are provided which are selectively.

first 'of which is tunable.

responsive to predetermined frequencies discycle and at least a major fraction of one per cent. of the derived intermediate frequency. Frequency-correcting meansare coupled to the freplaced above and below the predetermined fre-` quency b'y an amoimt of the `order of one kiloquency-discriminating means for adjusting the frequency-determining circuit to reduce the deviations of the second signal-carrier frequency -in a predetermined ratio of the order of 10:1.

The reactive constants of the frequency-determining circuit, the reactive constants of the frequency-discriminating ,means, vand the response characteristic of the frequency-correcting means Aare so related that deviations of the second signal-carrier frequency effected by mlstuning of the frequency-changing means beyond a predetermined amount of the order of, but less than, the predetermined frequency separation effect relaxation of the frequency-correcting means.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description,

taken in connection with the accompanying' drawings, and its scope will be pointed out 4in the appended claims.

In the accompanying drawings, Fig. l is acircuit diagram of a complete superheterodyne radio receiver, partly schematic, embodying the present invention, while Figs. 2, 3 and 4 are graphs representing certain operating characteristics of the receiver,` to aid in the understanding ofthe invention.

Referring now more particularly to Fig. 1

of the drawings, there is shown schematically aI superheterodyne receiver embodying the present invention in a preferred form. In general, the receiver includes a tunable radio-frequency ampliiier III connected to an antenna 8 and ground 9. Connected in cascade with the radio-frequency amplifierl I0, in the order named, are la first frequency changer II, a first intermediatefrequency amplifier I2, a second frequency changenindicated generally at I3, a second intermediate-frequency amplifier ",a detector and Automatic amplification, control bias voltage developed at the AVC. supply I5 may be applied by way of suitable resistors, as shown, to one og more of the radio-frequency amplifier, frequency changer, and first and second intermen diste-frequency amplflier, in accordance with established practice. It will be understood that the several parts of the system which are illustrated schematically in the drawings may be con- I ventional in their construction and operation, the details of which are well-,known in the art, ,rendering description thereof unnecessary herein.

Neglecting for the moment-the particular operation of theparts of the systeminvolved in the present invention, which is hereinafter described in detail, the system abovedescribed lncludes all the features of a conventional triple detection superheterodyne receiver. The operation of such a superheterodyne receiver being well understood in the art, detailed explanation thereof is, therefore, deemed unnecessary. In

brief, however, a desired modulated-carrier signal intercepted by the antenna is selected and amplified in the radio-frequency ampliner Il and converted by the -flrst frequency changer II to a secondmodulated-carrier signal 'at a' first predetermined intermediate carrier frequency. This second signal is selected and amplified in the ilrst intermediate-frequency amplifier Il and is 'thereupon converted by the secondfrequency changer Il to a third modulated-carrier signal at a second predetermined intermediate carrier frequency. The third signal is selected and ampliiied in the second intermediate-frequency amplifier Il and is then converted by the detector i5 to audio frequencies of modulation which are amplified in the audio-frequency amplifier It and reproduced by the sound reproducer II. Biasing potentials, developed by the AVCs supply in a well-understood manner, are supplied to control the gain of the radio-frequency amplier, first frequency changer, and first intermediate-frequency amplier, to maintain the amplitude of the signal output of the amplifier I4 within'a relatively narrow range for a wide'range of received signal amplitudes.

Referring now more particularly to the portion of the system in connection with which the present invention is employed, the frequency changer i3 comprises a pentagrid loscillator-modulator tube I8 having its signal control grid-cathode circuit coupled to the output circuit Vof the rst intermediate-frequency amplifier il, a suitable biasing resistor I! and by-pass condenser 2l being included in the cathode circuit. An oscillation circuit including an inductance II and a condenser 22 in parallel is connected between the first or oscillator grid of the tube Il and the cathode thereof by way of suitable coupling condensers 2 3 and 2l. be described, s. resistor 2l ulncluded in the mductance arm of the osciliationcircuit. Ii suitable leak resistor is connected between the oscillator grid and the cathode of the tube. A

feed-back inductance, 21 is connected between the second grid or oscillator anode of the tube -Il and the cathode thereof by way of .a coupling condenser 2l and the above-mentioned condenser 24 and is inductively coupled lto the inductance 2l of the oscillation circuit. The output or anode circuit of the tube Il is suitably. connected to thev input circuit of the second intermediate-tref quency amplifier Il, as shown.

Proper operating voltages are supplied to the electrodes of the tube II from-sources as' indicated by ll-Sc for the screen supply and +B for the output and oscillator anodes, a suitable resistor 29 being included in the supply lead to the oscillator anode.l A shield is preferably provided for the oscillation circuit, as indicated by the broken lines 3l. The selective modulated-carrier signal receiver thus comprises a tunable frequency-changing means including a frequencydetermining circuit-for derivingl any s'clected modulated-carrier signal within a de frequency range of such signals having a predetermined carrier-frequency separation another modulated,- carrier signal havinga carrier normally of a different predetermined fequency. 'Ihis predetermined Vfrequency in accordance with the invention is of an order of magnitude not greater than 100 kilocycles. The derived carrier wave is' While, in accordance with the present inven- For a purpose presently to cream this tube is connected across the resistor 25of the oscillation circuit by way of coupling coudenser 32 and by-pass condenser 24. The anode circuit of the tube 3| is connected across the .anode circuit of the tube.

'entire oscillation circuit. operating voltage may be supplied to the anode of the control tube, as

shown, from the source as indicated at +B, by4

way of resistors 34 and. 25 and the inductance 2|, a suitable voltagebeing supplied for the screen grid of the tube from the source as indicated by +Sc. The suppressor grid is connected to the cathode and the latter electrode is suitably biased,

as indicated. by a battery 34a.

For developing the control bias voltage for the second predetermined carrier frequency by condensers 36, and the other circuit including an inductance 31 tuned to a frequency adjacentto and above the second predetermined carrier frequency by condensers 33. The two discriminator circuits arefindividually connected to a pair of rectiilers, comprised in a .double diode. tube 33, provided with suitable load circuits comprising resistors 4l, with by-pass condensers 4I, as shown. The load circuits are connected inseries, with their unidirectional voltages opposing. One terminal of the pair is grounded. and the other is connected to the control grid ofthe tube 3i by way of series resistors 42 and 33. The resistor 42 and condenser 43 serve to filter out the modulation components which are pres# ent in the output of the diode circuits and to provide a proper time constant for the control circuit. There is thus provided for the system frequency-discriminating means selectively responsive to predetermined frequencies displaced above and below the derived predetermined intermediate frequency,` in this case the second .lintermediate carrier frequency.

For coupling the discriminating circuits 35, 35 and 31, 33 to the output ofthe .amplifier- I4, there is provided an amplifier -tube 44 having its grid-cathode'circuit connected to the output cir' cuit of the amplifier i4 by way of a suitable coupling condenser 45. A leak resistor 45a is connected between the control grid and cathode of the tube 44, and a high impedance, comprising an inductance 45 tuned to the second predetermined frequency by a condenser 41, is included in the Suitable operating voltages are supplied to the screen and anode of this tube fromthe sources indicated at +Sc and +B, respectively, the latter by way of a lter resister 43, a by-pass condenser 4I also being in- .cluded in the anode circuit, as shown.

For coupling the output circuit 45, 41 to the pair-of discriminator circuits 35, 35 and 31, 38,

, there is provided a link circuit including in series a high impedance winding 5l relatively closely coupled to the winding 45 and a pair of low impedance -windings 5I and 52 relatively closely coupled to the windings 35; and 31, respectively, and grounded at their junction. The tuned circuits 35, 35 and 31, 3l and their respective coupling coils are individually shielded, as

indicated. The relative proportions of the various elements of the frequency-control means will be described in connection with the operation of -thesystem Referring now to the operation of the system, when any signal is being received, voltage of the third signalV frequency is supplied to the input tube is returned directly to the cathode through 'circuit-of the tube Il.4 Since the grid 0f thisy the leak resistor 45a, and 'is isolated from any other direct-current path by the condenser. 45,

whenever any signal is applied to the grid a negative grid bias voltage is developed which increases' with increasing signal input-amplitude. Thus, 4the mutual conductance of tube 44, and hence .its effective amplification, is'greatest for weak signals and decreases as the signal strength increases. Since the plate circuit of the tube 44 includes the resonant circuit "45, 41l which is designed with a relatively high LC. ratio and is tuned to the second predetermined frequency, Y.

thereby aifording ahigh impedance at this and adjacent frequencies, the plate circuit overloads or operatesbeyond both its upper and lower cutoiI limits' for all usable signals applied to the grid during normal operating conditions of the system. Thus, whenever a useful signal is being received, a` substantially constant voltage is obtained across the circuit `46, 41 regardless of.

variations in the amplitude of the signal, and

this voltage is supplied by way of the link circuit;Y

5I, 5I, 52 to the pair of circuits 35, 36 and 31! 38. As will bexdescribed hereinafter, in order to obtain the desired frequency control it is necessary that the selective circuits 35, 35 and 31, 33

Y lpled with respect to each other. Hence, besides being shielded, as mentioned above, the circuit elements are so proportioned as to maintain the Selective circuits substantially electrically independent from each other. More particularly, the impedance of the winding 50 is very large asA compared with the impedances of the windings 5| and 52. For example, in the preferred embodiment the impedance of the winding 50 is of the order of 2500 times that of each of the' windings 5I and 52; While this arrangement, ofcourse, does not provide an eiiicient voltage transfer, a high step-down voltage ratio from theoutput circuit of the tube .44 to the input circuits of the doublediode tube 39 is needed and this ratio is obtained with the arrangement described.

"'"The unidirectional' voltages developed across the resistors 40 are of opposite polarity so that their diiference varies positively or negatively in accordance with deviations of the second predetermined carrier frequency toward the resonant frequency of one orthe other of the circuits 35, 35 and' 31, 33, and this-voltage is ap- Y plied to the control grid of-the tube 3l, as mentioned above. Since the input circuit of the tube 3| is connected across the resistor 25, which is 'loV the voltage across the oscillation circuit by 90v and the tube 3|, therefore, simulates a low power factor inductance.

The bias voltage applied to the grid of tube 3| from resistors lll, mentioned above, varies in accordance with the deviations of the frequency of the third signal, or second intermediate-frequency carrier, from its predetermined ornormal frequency and thus controls the mutual conductance of the tube 3l and, hence, the amplitude of the lagging current supplied to the oscillation circuit by this tube. Thus, the apparent inductance and resonant frequency of the oscillation circuit are varied directly in accordance with the frequency deviations of the second intermediatecarrier frequency from its nor- .mal predetermined value.

l Since the frequency of the third signal, which is developed by the frequency changer I3, is equal to the sum or deviationof the order of :1`is procured. In

other words, mistuning adjustments of the first frequency changer, which would tend to result .in deviations of the third signal carrier frequency to an extent equal to such mistuning, are substantially compensated, the deviations of the third signal carrier frequency being reduced to approximately one-tenth this amount.

In order that the controlslstem shall be effective to maintain the receiver accurately adjusted for reception of any desired signal only so long as the mistuning does not equal or exceedv the frequency separation of adjacent signals, so that itwill not be possible completely to skip over an adjacent signal, it is necessary that, while the control action shall be effective for all deviations of the second predetermined 'frequency within predetermined limits, for examplei'lOO cycles, resulting from a mistunlng adjustment of the first frequency changer of approximately 4 -7 kilocycles, the control action j shall be ineffective or relax, or preferably reverse, for deviations beyond these limits. 'I'hese relations are obtained by virtue of selection of the resonant frequencies o`f the selective circuits 35, 36 and 3l, 38 closely adjacent to and above and below the second predetermined carrier frequency; for example, they may be tuned to frequencies 700 cycles below 'and above the second predetermined frequency. Referring to Fig. 2, here frequency deviations of the third 'signal carrier from its normal predetermined frequency, in kilocycles, are indicated by the abscissae and the ordinates represent control bias voltages developed by the control means and applied to the tube 3l. The curve 53 thus represent's the frequency-response characteristics of the control system. It will be seen that this curve includes a relatively steep portion for frequencies close to the normal predetermined frequency and that the peaks of the curve are at frequencies corresponding to the resonant frequencies of the pair o f selector circuits. the portions of the curve beyond the peaks returning toward the zero bias condition, that is, representing a reversal in the control action. Thus, when mistuning of the first frequency changer results in a deviation of the frequency ofthe third signal carrier to one side or the other of its predetermined frequency, the control action is effective to reduce these deviations in alarga ratio as long as they are within the limits represented -by the peaks of the curve. When. however, the mistuning is to such an extent, for example more than .i7 kilocycles, as to 'cause deviations of the third signal carrier beyond the limits indicated by the peaks of the curve, the control action, as shown by the outerportions of the curve, in effect reverses;l that is, the selective circuits become increasingly less responsive. This latter condition, of course, results in a decrease in the correction of oscillation frequency, thereby causing still greater deviations in the third signal carrier frequency, and these greater deviations, ofcourse, result in still less responsiveness of the selective circuits. Hence, a regenerative or snap action of the control means is effected by deviations beyond the predetermined limits, due to the effective reversal of the normal control action. Beyond these limits, therefore, the control system causes a readjustment of the frequency of the oscillation circuit toward its normal value. Under these conditions, .the first frequency changer is ad,

justed for receiving the next adjacent'carrier of the range lto derive therefrom a rst intermediate-frequency signal at approximately its normal frequency, and the second frequency changer andthe control system will operate in the manner described aboveto derive a second intermediate frequency and to maintain it withis, the displacement of Vthe resonant frequency' of each of the frequency-discriminating circuits is an amount of the order of one kilocycle and at least a major fraction of one per cent. of the second intermediate frequency. This is effected by virtue of relationships between the reactive constants'of the frequency-determining circuit,

whereby a very low secondV intermediate frequency is obtained, kilocycles in the illustrated embodiment, and the reactive constants of the frequency-discriminating means which determine their detuning with respect to the second intermediate frequency. i700 cycles in the present case. It will be here noted `that if a 450 kilocycle carrier were used the same frequency separation of the peaks would be less than 0.5%. Thus, employing the/lower intermediate freque'lcy,`v a given percentage error in the tuning of the two selector circuits results in a substantially smaller error in the frequency separation of the resonant frequencies of these circuits. l

with respect to the intermediate carrier frequency. y I' In order to obtain the desired range of control and the maximum sensitfifvity-ffor-agiven permissible range of deviations, it is necessary' that the control be responsive to'allfgnieviations variations in the mutual conductance of the tube n 3l are eifective to shift the resonant frequency i the eillation circuit, as above explained. In Pig.3.thecurve Slclearlyshowstheresponse dihetubeLthatisitseifectincontrolling the-resmant frequency of the oscillation circuit 'l' lnaccordancewithchangesingridbiasvoltge applied thereto. "lhechanges in the gridbias volage are by the abseissae of the mure and the ordinates reprent the resultant v'clnnges in frequency vin kilocycles of the oscilla- I in lik. 2. In other wordsthe control bias ad- ;lmtingmeans is so relatedA to themutual conof the control tube that the lias voltages .applied thereto, corresponding tu maximum permisible deviations of the carrier 'l Ircqmcyofthesecondsignalwayseifectopentimofthecontroltubewithinboth thellppel 'lhe ultimate eilecis of Iare clarly shown in Fig. 4,*wherein tuning misad- 'I jllstlnentsoftheiirstfrequeneychangeiiinkilo- -cycles,areindicatedbythe,andthere sultantfrequencydeviationsofthethirdsgnal f eaerfrequencyfrnthedesiredsecondpredeterminedfrequencyareindicatedbythe bannieres. The-cuve ssshowsthe frequencyj deviations of the second signal and, if nofrelllcncy control is employed,also those of the Inthissameflgurecurveishows theremlls obtained when the frequency control l of the prent invention is employed. It will be Yuseful intensities, Athat is, signals which are of suillcient amplitude to provide the standard output for the receiver, while for weaker signals theY frequency control means become decreasingly effective.

superheterodyne arrangement providing a rela-A i tively xed oscillator frequency which is subjccted to the frequency control. the samecontrol. action is obtained regardless of the first signal carrier frequency to which the system is tuned. Furthermore, due to the proporloning and arrangement of the various elements of the control circuits, the control action is substantially independent of ordinary variations in the operating voltages utilized. A While there has been described what is at Y present considered the preferred embodiment of u the invention, it will be obvious to those' skilled Moreover, by virtue Yof the double in :heeft um venom changes nndfmodineetions may be made therein without departing from the invention, and it is, therefore, aimed inthe appendedclaimsto coverallsuch changes and modifications as fall within the true spirit and scope -of the invention.

What is claimed is:

l. A selective modulated-carrier signaling sys-` tem comprising tunable freqncyllleBDS including 2- frequency circuit for deriving from any selected modulated-carrier sgnal 'within a wide frequency range of suchsignals having aA p carrier-frequency separation a second modulated-carrier signal having a carrier normally of a predetermined frequency of an order of magnitude in kilocycles not greater than 100 but subject to deviations wlthin narrow limits upon mistuning of said' frequency-changing means, frequency-discriminating means selectively responsive to 'predetermined frequencies displaced -above and below said`predetermined frequency by an amount of the order of one kilocycle andoat least amajor fraction "of one per cent. of said intermediate frequency, frequency-correcting means coupled to said frequency-discriminating means for adjusting said frequency-determining circuit to reduce the `deviations of said second signal carrier frequency in a predetermined ratio of Vthe order of 10 tol, the'reactive constants ofsaidfre quencydetermining circuit, the reactive con-v stanls of .said means, Yand the response characteristic of said frequency-correcting means beingso related that deviations of said second signal carrier frequency effectedbymistuningofsaidfrequency-changmg meansbeyondapredeterminedamountofthe.

order ofbutiesethnnsaidpfrequency separation eifect relaxation of said frequency-correcting means.

2. YA selective modulated-carrier systenicomprising tunable frequencymeans including a frequency circuit for deriving from a selted modulated-carrier signalwithinawidefrequencyrangeofsuch Signals' having a p Ycarrier-frequency separation a second modulated-carrier signal having a carrier normally of apredetermined frequency of an order of magnitude in kilocycles not greater than 100 but subject to deviations within narrow limits upon mistuning of. said frequency-changing -means, frequency-- determining means selectively responsive to predetermined frequencies above and be low-said predetermined frequency by an amount of the order o f one kilocycle and at least a major fraotion of. one per cent. or said intermediate frequency, frequency-correcting means coupled to said frequency-discriminating means for adjusting said frequency-determining circuit to control the extent of the deviations of said sec-Y ond signal carrier frequency in a predetermined ratio to the extent of said of the order of 10 to l, the reactiveconstants of said frequencg-determining circuit, the reactive constants 'of said frequency-discriminating means, and the response characteristic of said frequencycorrecting means being so related vthat deviations of said second signal carrier frequency due to mistuning of said frequency-changing means within a predetermined amount, of the order of but less than said predetermined frequency sepby mistuning beyond said predetermined amount effect control of said correcting means to tune said frequency-determining circuit to derive a second signal of said predetermined frequency from the next adjacent signal in said range.

3. A modulated-carrier signaling system comprising a tunable rst frequency changer for deriving from any selected modulated-carrier signal within a wide frequency range of such signals having a lpredetermined `carrier-frequency separation a second modulated-carrier signal having a carrier normally of a rst predetermined frequency, a fixed-tuned second frequency changer including a frequency-determining circuit for deriving from said second signal a third modulated-carrier signal having a carrier normally of a second predetermined frequency of an order of magnitude in kilocycle's not 'greater kthan 100, said second and third signal carrier frequency-determining circuit to reduce the de-- viations of said third signal carrier frequency in a predetermined ratio ofthe order of 10 to l, said second predetermined frequency, the reactive constants of said frequency-determining cireuitythevreactive constants of said frequencydiscriminating means, and the response characteristic of said frequency-correcting means being so related that deviations of `said third signal carrier frequency effected by mistuning of said first frequency changer beyond a predetermined amount of the order of but less than 'said predetermined frequency separation effect relaxa tion of said correcting means.

4. A modulated-carrier signaling system comprising a tunable iirst frequency changer for deriving from a selected modulated-carrier signal within a wide frequency range of such signals having a predetermined carrier-frequency separation a second modulated-carrier signal hav-v ing a carrier normally of a first predetermined frequency, a fixed-tuned second frequency changer including a tunable oscillation circuit for deriving from said second signal a third modulated-carrier signal having'va carrier normally of a second predetermined frequency of an order of magnitude in kilocycles not greater than 100, said second and third signal carrier frequencies being subject to deviations within narrow limits upon mistuning of said first frequency changer, frequency-discriminating means selectively responsive to predetermined ,frequencies displaced above and below said second predetermined frequency by an amount of the order of one kilocycle and at least a major fraction of one per cent. of said intermediate frequency, frequency-correcting means coupled to said frequency-discriminating means for adjusting the resonant frequency of said oscillation circuit in accordance with said deviations to reduce the deviations of said third signal carrier frequency with respect to the deviations of said second signal carrier frequency in a predetermined ratio of the order of 10 to 1, the reactive constants of said oscillation circuit, the reactive constants of said frequency-discriminating means, and the response characteristic of thefrequency-correcting means being so related that deviations of said third signal carrier frequency effected by mistuningvof said first frequency changer beyond predetermined amount of the rder of but less than said predetermined frequ cy separation effect relaxation of said correcting means.

NELSON P. CASE. 

